Several national governments, including the United States (U.S.) of America, are presently developing a terrestrial position determination system, referred to generically as a global positioning system (GPS). A GPS is a satellite-based radio-navigation system which is intended to provide highly accurate three-dimensional position information to receivers at or near the surface of the Earth.
The U.S. government has designated its GPS the "NAVSTAR." The NAVSTAR GPS is expected to be declared fully operational by the U.S. government in 1993. The government of the former Union of Soviet Socialist Republics (U.S.S.R.) is engaged in the development of a GPS known as "GLONASS". Further, two European systems known as "NAVSAT" and "GRANAS" are also under development. For ease of discussion, the following disclosure focuses specifically on the NAVSTAR GPS. The invention, however, has equal applicability to other global positioning systems.
In the NAVSTAR GPS, it is envisioned that four orbiting GPS satellites will exist in each of six separate circular orbits to yield a total of twenty-four GPS satellites. Of these, twenty-one will be operational and three will serve as spares.
Each GPS satellite will orbit the Earth approximately once every 12 hours. This coupled with the fact that the Earth rotates on its axis once every twenty-four hours causes each satellite to complete exactly two orbits while the Earth completes one revolution.
The position of each satellite at any given time will be precisely known and will be continuously transmitted to the Earth. This position information, which indicates the position of the satellite in space with respect to time (GPS time), is known as ephemeris data.
In addition to the ephemeris data, the navigation signal transmitted by each satellite includes a precise time at which the signal was transmitted. The distance or range from a receiver to each satellite may be determined using this time of transmission which is included in each navigation signal. By noting the time at which the signal was received at the receiver, a propagation time delay can be calculated. This time delay when multiplied by the speed of propagation of the signal will yield a "pseudorange" from the transmitting satellite to the receiver. A pseudorange computed in this manner is called a "code" pseudorange.
The range is called a "pseudorange" because the receiver clock may not be precisely synchronized to GPS time and because propagation through the atmosphere introduces delays into the navigation signal propagation times.
These result, respectively, in a clock bias (error) and an atmospheric bias (error). Clock biases may be as large as several milliseconds.
Using these two pieces of information (the ephemeris data and the pseudorange) from at least three satellites, the position of a receiver with respect to the center of the Earth can be determined using passive triangulation techniques.
Triangulation involves three steps. First, the position of at least three satellites in "view" of the receiver must be determined. Second, the distance from the receiver to each satellite must be determined. Finally, the information from the first two steps is used to geometrically determine the position of the receiver with respect to the center of the Earth.
Triangulation, using at least three of the orbiting GPS satellites, allows the absolute terrestrial position (longitude, latitude, and altitude with respect to the Earth's center) of any Earth receiver to be computed via simple geometric theory. The accuracy of the position estimate depends in part on the number of orbiting GPS satellites that are sampled. Using more GPS satellites in the computation can increase the accuracy of the terrestrial position estimate.
Conventionally, four GPS satellites are sampled to determine each terrestrial position estimate. Three of the satellites are used for triangulation, and a fourth is added to correct for the clock bias described above.
For a more detailed discussion on the NAVSTAR GPS, see Parkison, Bradford W. and Gilbert, Stephen W., "NAVSTAR: Global Positioning System--Ten Years Later," Proceedings of the IEEE, Vol. 71, No. 10, October 1983; and GPS: A Guide to the Next Utility, published by Trimble Navigation Ltd., Sunnyvale, Calif., 1989, pp. 1-47, both of which are incorporated herein by reference. For a detailed discussion of a vehicle positioning/navigation system which uses the NAVSTAR GPS, see commonly owned U.S. Pat. Appl. Ser. No. 07/628,560, entitled "Vehicle Position Determination System and Method," filed Dec. 3, 1990, which is incorporated herein by reference.
NAVSTAR GPS envisions two modes of modulation for the carrier wave using pseudorandom signals. In the first mode, the carrier is modulated by a "C/A signal" and is referred to as the "Coarse/Acquisition mode". The Coarse/Acquisition or C/A mode is also known as the "Standard Positioning Service". The second mode of modulation in the NAVSTAR GPS is commonly referred to as the "precise" or "protected" (P) mode.
The P-mode sequences are held in secrecy by the United States government and are not made publicly available. The P-mode is intended for use only by Earth receivers specifically authorized by the United States government. Thus, the P-mode modulated data is generally not available so that many GPS users must rely solely on the GPS data provided via the C/A mode of modulation. This relegates most users to a legs accurate positioning system.
The clock and atmospheric errors discussed above add to the inaccuracy of the positioning system. Other errors which affect GPS position computations include selective availability (accuracy corruption intentionally introduced by the U.S. government for reasons of national security), receiver noise, signal reflections, shading, and satellite path shifting (e.g., satellite wobble). These errors result in computation of incorrect pseudoranges and incorrect satellite positions. Incorrect pseudoranges and incorrect satellite positions, in turn, lead to a reduction in the precision of the position estimates computed by a vehicle positioning system.
Methods are available for compensating or correcting for many of these errors. For example, one method uses a differential system (discussed below) to produce a linear bias for each pseudorange (i.e., a bias is calculated for each satellite). A base station, having a fixed, known position, computes a pseudorange to each satellite. The base station further computes a distance between its known position and the position of each satellite (computed from the ephemeris data). By comparing the pseudorange to each computed distance, a pseudorange bias can be computed for each satellite. The pseudorange bias for each satellite can then be transmitted to the vehicle for use in the position estimate computations.
A differential GPS system produces precise position estimates. However, a base station is not always available to a receiver wanting to compute an accurate position estimate.
Accumulated delta range (ADR) or integrated doppler techniques may also be used to improve the accuracy of position estimates. ADR techniques sense changes in the phase of the carrier wave of a GPS navigation signal received at a receiver. The changes in phase can be related to a change in the line-of-sight distance (delta ranges) between the transmitting satellite and the receiver.
An ADR is computed by tracking the phase of the carrier wave of the GPS navigation signal. For example, as a satellite and a GPS receiver move away from each other, the increase in distance (i.e., the delta range) can be noted as a phase change in the GPS carrier wave. Because the GPS carrier wave is a continuous sinusoid, a delta range (DR) is not an absolute range. Rather, a delta range is a change in the range between the satellite and the receiver.
If an initial offset (cycle count) is known, the delta range can be used to compute an accumulated delta range. The accumulated delta range is an actual range estimation. Therefore, the accumulated delta range may be used in place of the code pseudorange. Alternatively, either the delta range or the accumulated delta range may be used in conjunction with the code pseudorange to compute a refined pseudorange. The accumulated delta range may also be called a "carrier" pseudorange.
GPS carrier phase techniques are described in detail in Dr. Jim Sennott (Dept. of Electrical & Computer Engineering, Bradley University, Peoria, Ill.) and Jay Spalding (U.S. Coast Guard, R&D Center Groton, Conn.), "Multipath Sensitivity and Carrier Slip Tolerance of an Integrated Doppler DGPS Navigation Algorithm," U.S. Department of Transportation, Transportation Systems Center, Cambridge, Mass., U.S. DOT University Contract DTRS-57-85-C-0090, and in Patrick Y. C. Hwang and R. Grover Brown, "GPS Navigation: Combining Pseudorange with Continuous Carrier Phase Using a Kalman Filter," Proceedings ION GPS-88, September 1989, pp. 185-190; each of which is incorporated herein by reference.
A problem with known ADR techniques is that cycle slips may occur. A cycle slip occurs when a receiver loses track of the phase of a carrier wave. This might occur, for example, due to shading of the receiver from direct line-of-sight reception from the satellite or from noise, either of which cause a momentary loss of receiver lock on the GPS carrier signal. When a cycle slip occurs, the accumulated delta range is lost (because the initial offset is no longer precisely known) and must be re-accumulated.
Many conventional navigation systems may not detect a cycle slips. If a cycle slip is not detected, then the incorrect cycle count will introduce error into the position computations. The conventional navigation system, i.e., the Kalman filter of the navigation system, will correct for the error, however, it may take tens of seconds before compensation for the error begins and several minutes before the error is substantially eliminated from the position estimates. These times are too long for real time positioning in many applications.
What is needed is a method for quickly detecting cycle slips.